1. Field of the Disclosure
The present invention relates generally to circuits that drive light emitting diodes (LEDs). More specifically, embodiments of the present invention are related to LED driver circuits that including triac dimming circuitry.
2. Background
Light emitting diode (LED) lighting become very popular in the industry due to the many advantages that this technology provides. For example, LED lamps have a longer lifespan, fewer hazards and increased visual appeal when compared to other lighting technologies, such as for example compact fluorescent lamp (CFL) or incandescent lighting technologies. The advantages provided by LED lighting have resulted in LEDs being incorporated into a variety of lighting technologies, televisions, monitors and other applications that may also require dimming.
One known technique that has been used for dimming is the use of a triac circuit for analog LED dimming or phase angle dimming. A triac circuit operates by delaying the beginning of each half-cycle of ac power, which is known as “phase control.” By delaying the beginning of each half-cycle, the amount of power delivered to the lamp is reduced and the light output of the LED appears dimmed to the human eye. In most applications, the delay in the beginning of each half-cycle is not noticeable to the human eye because the variations in the phase controlled line voltage and the variations of power delivered to the lamp occur so quickly. Although triac dimming circuits work especially well when used to dim incandescent light bulbs since the variations in phase angle with altered ac line voltages are immaterial to incandescent light bulbs, flicker may be noticed when triac circuits are used for dimming LED lamps.
LED lamps are typically driven with LED drivers having a regulated power supplies, which provide regulated current and voltage to the LED lamps from ac power lines. Unless the regulated power supplies that drive the LED lamps are specially designed to recognize and respond to the voltage signals from triac dimming circuits in a desirable way, the triac dimming circuits are likely to produce non-ideal results, such as flickering, blinking and/or color shifting in the LED lamps.
A difficulty in using triac dimming circuits with LED lamps comes from a characteristic of the triac itself. Specifically, a triac is a semiconductor component that behaves as a controlled ac switch. Thus, the triac behaves as an open switch to an ac voltage until it receives a trigger signal at a control terminal, which causes the switch to close. The switch remains closed as long as the current through the switch is above a value referred to as the holding current. Most incandescent lamps easily draw more than the minimum holding current from the ac power source to enable reliable and consistent operation of a triac. However, the comparably low currents drawn by LEDs from efficient power supplies may not be enough compared to the minimum holding currents required to keep triac switches conducting for reliable operation. As a consequence, conventional power supply controller designs usually rely on the power supply including a dummy load, sometimes called a bleeder circuit, in addition to the LEDs to take enough extra current from the input of the power supply to keep the triac conducting reliably after it is triggered. In general, a conventional bleeder circuit is external from the integrated circuit of the conventional power supply controller. However, use of the conventional bleeder circuit external to the conventional power supply controller requires the use of extra components with associated penalties in cost and efficiency.